A liquid-dispensing pistol-grip nozzle for fuel is connected by a filling hose to the liquid-dispensing machine or metering unit equipped with a delivery pump which can be turned on and shut off. The nozzle can comprise a valve housing with a spigot, a filling hose connector for connecting the nozzle with the filling hose, a fuel duct in the nozzle, an operating mechanism with a pistol-grip handle and if necessary a flow rate setting mechanism usually in the pistol-grip handle, an automatic shut off device and a safety device.
A valve seat can be located near the upstream end of the valve housing. An associated valve body loaded by a closing spring can be mounted in the valve housing and fitted with an axially slidable valve stem which cooperates with the seat. The valve stem is a hollow spindle in which is guided a slidable pull rod connected with the pistol-grip handle. The valve spindle and the pull rod are couplable and uncouplable by a locking mechanism belonging to the automatic shut off device.
The automatic shut off device can have a pressure equalizing chamber, a vacuum chamber with a filling state sensing duct and with a vacuum duct opening near the valve seat and a chamber-separating control membrane which operates the locking mechanism, the control membrane having a control membrane spring with which the shut off low pressure of the automatic shut off is set.
Further, the control membrane can be connected with flexible free play to a hollow piston shaft of a safety piston guided in the valve housing which is movable between a locked in position and a locked out position. On the side facing away from the control membrane the safety piston is acted upon by pressure in the filling hose connector communicated thereto by a pressure duct and is supported with a safety piston spring in the valve housing on the control membrane. The locked in position and/or the locked out position indicate configurations in which the locking mechanism is lockable into and/or lockable out of. In one position the valve cannot be operated because of failure of the fuel supply while in the other the fuel can be dispensed.
In the known liquid-dispensing nozzle (U.S. Pat. No. 4,331,187) the safety mechanism uses a ring which seals at the vacuum chamber. This mechanism impairs the reliability of operation when the soft flexible material used in the sealing ring changes its shape for example by swelling. Variations in the motion-direction-dependent friction are also troublesome.
In this aforementioned mechanism the safety device causes a reliable locking out when the delivery pump with open delivery valve is shut off or when an attempt is made to open the delivery valve after such a shut off. A shut off should be possible when a pressure exists in the filling hose and consequently also in the filling hose connector whose upper limit is determined by the loading of the delivery valve on the one hand and the pressure maintained in the switched off delivery pump on the other hand.
Consequently, additional steps were required in this known liquid-dispensing nozzle in order to attain the desired operation, namely a pressure relief for the filling pump hose and also the safety piston after shutting off the delivery pump. Suitable flow ducts were provided which lead to the spigot. The pressure relief thus causes a delivery of a residual quantity by the spigot which is frequently disturbing. Besides, the safety piston and the safety piston spring could not be conveniently designed for the full delivery pump pressure of the operating delivery pump, although it acts with the delivery pump turned on and the delivery valve closed with the pistol-grip handle in the closed position and the locking mechanism locked out.
Actually this pressure falls off a fraction for hydrodynamic reasons in the filling hose and in the filling hose connector when the fuel is delivered in a complete flow by the spigot in due course. In this operating condition a reliable locking out by the safety piston may not occur, since the safety piston spring must be weakened. That allows a haphazard emptying of the filling hose after shut off of the delivery pump which can occur inadvertently, e.g. by compressing it. That also allows a failure of the delivery system when the delivery pump because of a disorder is not brought to its full delivery pressure or for some reason the complete liquid flow does not reach the filling tube connector and consequently the automatic hydrodynamically controlled shut off device does not function for hydrodynamic reasons. In this case the liquid-dispensing nozzle may pose a danger since, for example, a vehicle tank to be filled can uncontrollably be caused to overflow.